Generally, communication systems in which numerous transceiver terminals share the total available transmission capacity must employ some form of contention-access protocol to achieve an efficient use of that capacity. Examples of commonly employed contention protocols used in communication systems involving bursty traffic, i.e. short sporadic transmissions, include ALOHA, slotted ALOHA, and CSMA (carrier sense multiple access).
More specifically, ALOHA was originally developed for packet radio networks. In an ALOHA arrangement, whenever a terminal wishes to send a signal (i.e., a packet or frame), that terminal simply begins transmitting the signal. The terminal then listens for either its own transmission, or a return acknowledgement sent by the destination terminal, and if the transmission or acknowledgement is not received within a certain period of time, the transmitting terminal will retransmit the signal.
Slotted ALOHA was developed as a modification to "pure" ALOHA to improve system utilization and efficiency. In a slotted ALOHA arrangement, time on a channel is divided into uniform slots, with transmission only being permitted to begin at the start of a slot. A central clock is typically used to synchronize all competing stations. As with pure ALOHA, slotted ALOHA arrangements either listen for their own transmission as a form of acknowledgement, or utilize a return acknowledgement signal sent by a destination source as an indication of non-collision of the transmitted signal.
CSMA was developed to improve utilization efficiency of a communication system over that which is achieved with ALOHA. In a CSMA arrangement, a terminal that wishes to transmit must first listen to the transmission medium to determine if there are any ongoing transmissions. A terminal can only transmit if the medium is idle, and if the medium is detected as being busy, the terminal must wait until the medium becomes idle. If an acknowledgement is not received from the destination terminal, the transmitting terminal will assume a collision has occurred and begin retransmitting the signal.
While such contention protocols improve utilization efficiency, problems arise in multiple-path or multiple-beam communication systems because a source and destination terminal can reside on different paths or beams. Thus, a transmitting terminal may not be able to receive any of its own transmissions. As a result, conventional contention protocols such as ALOHA or CSMA will not provide a satisfactory level of access sharing and system utilization. In addition, the use of an acknowledgement signal sent by a destination terminal upon proper receipt of a transmitted signal can double the delay time for detecting a collision, and reduce terminal throughput.